Time signals with periodic components, for example sinusoidal components or pulse sequences, occur frequently in technical applications, for example in telecommunication, in range finding applications with radar or echo, in structural dynamics, in speed measurement with the aid of laser-Doppler anemometry or with the laser-2-focus method as well as in transit-time measurement by means of ultrasonic methods. From the time signals, then, any existing fundamental frequency or a pulse interval is determined, with the aid of which the parameter sought can then finally be determined.
A method of the type described at the beginning is known from applicant's own, unpublished application. According to the available rows of the light modulator, the time signal is divided into individual sections and digitized. This section of the digitized time signal is then represented on the light modulator by the individual elements of the light modulator being switched to translucent or non-translucent. This light modulator is then transilluminated with coherent light and the light is focused in the Fourier plane. The Fourier-transformed image, that is a pattern of dots, is then produced in the Fourier plane, the spacing of the dots neighboring a central dot being proportional to the frequency of the time signal. What is advantageous about this known method is that the Fourier-transformed image of the section of the time signal is represented virtually without delay. Consequently, a very fast evaluation is possible. What is disadvantageous, however, is that the time signal can only be represented sectionally on the light modulator, since the latter only has a finite number of columns. On account of this, dead times occur in the case of prolonged and high-frequency time signals, i.e. this means that not the complete time signal can not be evaluated. These dead times are to be explained by the fact that evaluation first has to take place after each sectionally represented time signal in order subsequently to be able to represent a new section of the time signal on the light modulator. The timing signal arising in this time is lost. It is also disadvantageous that no evaluation of long time signals is possible, in order to obtain an averaged result, with this known method. A simultaneous evaluation of time signals which originate from different signal sources, such as is the case for example in stationary vibration tests with various vibration pickups, is also not possible by optical means. In order to obtain such an average value, it is necessary to carry out the averaging in a possibly on-line computer later. The simultaneous evaluation of a number of time signals is not possible. Also, the simultaneous recording and representation of the frequencies of a number of time signals is not possible.
It is also known to evaluate the time signals with an FFT analyser. The fast-Fourier transformation of the time signal is carried out in such a commercially available FFT analyser What is disadvantageous is that such FFT analysers have to manage with a limited number of interpolation points, usually about 512 in the case of not very fast devices, about 64 in the case of fast devices, meaning that the fundamental frequency is no longer detectable if there is considerable noise, i.e. a low signal-to-noise ratio. What is also disadvantageous is that the computing time is relatively long. Where high frequency signals are involved, sections which are not recorded occur between the individual sections of the time signal in the case of these FFT analysers as well. The dead time arises mainly from the time which the computer requires for the evaluation or for the FFT analysis. There is, of course, the possibility of largely eliminating this dead time by using a plurality of FFT analysers, but this entails a considerable expenditure. Furthermore, such FFT analysers are relatively expensive, meaning that for this reason alone normally a plurality of FFT analysers cannot be used.